Scientists have discovered a substance that can temporarily block the development of sperm without altering testosterone levels and without causing unwanted side effects.

They hope human trials of a new contraceptive for men based on their discovery could begin within a few years.

Other versions of the 'male Pill' are already in development but work by altering hormone levels in the man's body.

In trials so far these have produced no worrying side effects - however scientists think men may still worry about whether introducing female hormones could harm their virility in some way.

The new approach would therefore avoid this problem. The common perception is that few women would actually believe a man who said he was on the Pill.

However a study published in the British Medical Journal in 2000 found that only two per cent of women said they would not trust their partner to take a male Pill.

Several teams of scientists have therefore been working on trying to develop an equivalent of the female Pill for men.

Until now this has involved using hormones to try to prevent sperm production.

But the new approach reported in the latest edition of the journal Nature Medicine involves a substance called adherin.

It works by interfering with the way cells in the testes help nurture the development of mature sperm.

It blocks the normal bond which develops between immature sperm and tissue called Sertoli cells.

This then stops the cells developing into mature sperm which are capable of fertilising an egg.

A study found that when given to rats, sperm production dropped to such low levels as to render them infertile.

However once the drug was stopped, the animals' sperm production soon returned to normal.

Lead researcher Chuen Yan Cheng of the Population Council in New York said this would be vital for any kind of pill to become acceptable to men.

"If you give the consumer the peace of mind that their fertility will be restored and that their hormones are not going to be affected, they may feel that it is safer to use this contraceptive," he said.

Adherin on its own is known to have toxic effects on the body, but the team overcame this problem by ensuring it would only attach to one cell in the body - the Sertoli cells.

The team tested it on vital organs such as the kidneys, liver, heart and brain, and found it had no harmful side effects.

Dr Cheng and his team hope their finding will lead to a male contraceptive - although say it is likely to be an implant rather than a pill as the tablet would be broken down in the digestive tract.

Dr Cheng said the approach taken by other researchers of using hormone-based contraceptives was not wrong. However he said it was important to give people a choice.

Earlier this year researchers working on a hormonal-based male contraceptive said an implant or injection could be available within five years.

Another study, published in the Lancet found that men given a hormonal-based contraceptive drug saw their sperm count return to normal just over three months after they stopped using it.